Size coding of alternative responses is sufficient to induce a potentiation effect with manipulable objects

The mere perception of manipulable objects usually grasped with a power-grip (e.g., an apple) or a precision-grip (e.g., a cherry) potentiate power-grip- and precision-grip-responses, respectively. This effect is seen as to be driven by automatic access of the representation of manipulable objects that includes a motor representation of usually performed grasping behaviors (i.e., the embodied view). Nevertheless, a competing account argues that this effect could be due to an overlapping of size codes used to represent both manipulable objects and response options. Indeed, objects usually grasped with a power- and a precision-grip (e.g., an apple vs. a cherry) could be coded as large- and small-objects, respectively; and power- and precision-grip responses as large- and small-responses, respectively. We conducted 4 experiments to test this hypothesis. In Experiment 1, the response device usually used in studies reporting a potentiation effect is fixed horizontally (the grasping component of responses was removed). We instructed participants to press the small-switch with their index-digit and the large-switch with their palm-hand. In line with the size-coding-hypothesis, responses on the small-switch performed with the index-digit led to shorter RTs when objects usually associated with a precision-grip (e.g., a cherry) were presented compared to objects usually associated with a power-grip (e.g., an apple). A reverse pattern was obtained for responses on the large-switch performed with the palm-hand. In Experiments 2, 3 and 4, we went further by investigating which factors of Experiment 1 allow the size coding of responses: the size of switch and/or the size of the effector part used. Data confirmed the critical involvement of the size of switches and the possible involvement of the size of the effector part used. Thus, data support the possibility that the potentiation of grasping is due to a compatibility/incompatibility between size codes rather than involving motor representations of usually performed grasping behaviors as advocated in several embodied views. Moreover, data support the possibility that responses are coded thanks to a size code that extends the Theory of Event Coding.